It is known that the immobilization or entrapment of biological material may be obtained by a siliceous layer deposited on the surface of a matrix holding a dispersion of said biological material such as, for example, enzymes, bacteria, yeast, plant or animal cells, hereafter named “biomass”.
Such siliceous layer has to possess the following features:    a) immobilize the biomass preventing the release of the biomass from the matrix into the liquid medium of the bioreactor,    b) maintain the viability of the biomass allowing the controlled and defined transport between the immobilized phase and a culture medium, and    c) be biocompatible, so that body organs or fluids of living animals tolerate the contact with the immobilized biomass.
It is known a process for immobilizing a biomass by a sol-gel process, performed in solution starting from metal or silicon alkoxides in liquid phase.
However, the above process has several drawbacks such as, for example,:    a) it produces alcohols which are toxic in a number of biological systems,    b) the pH requirements may be unsuitable for living cells or may promote the denaturation of some bioactive enzymes,    c) the required stirring operations may be detrimental for cell lines sensitive to fluid-mechanical stresses,    d) some experimental procedures are incompatible with the required sterile environment, and    e) it is not applicable to biomasses requiring a definite structural organization to avoid loss of function.
In order to overcome the above problems, it has been suggested to immobilize the entrapped biomass within the matrix by means of a process which implies use of a gas medium carrying silicon alkoxides.
In particular, U.S. Pat. No. 5,998,162 describes plant cells immobilized in a siliceous layer obtained on the cell surface by reaction with gaseous silicon alkoxides. The immobilized cells are not released in solution over a period of six months and maintain their viability while producing secondary metabolites. Immobilized cells are used for pharmaceutical applications.
WO 97/45537 describes animal cells entrapped by a siliceous layer directly deposited on the cell surface by reaction with gaseous silicon alkoxides. The process is not toxic and viable cells are not leached by the medium and do not proliferate with medium invasion. Entrapped cells are used for bioartificial organs and for desired products.
However, it has been observed that, in some cases, the direct deposition of the siliceous layer on the cell surface reduces the effectiveness and function and that the body organs and/or the fluid of living animals were contaminated and the vitality of the viable biomass was quenched.